The present invention relates to a jib stretching and folding device for use in a jib down-folding type crane or a jib side-folding type crane.
Referring now to FIG. 5 showing a conventional jib side-folding type crane which holds a folded jib 2 having a left and a right beams (the terms "left" and "right" will be hereinafter defined as direction as viewed from an operator cab under the stretched position of the jib) on one lateral side of a telescopic boom 1 with the left and the right beams vertically aligned (upright posture,) the jib 2 can be stretched through the following three steps:
1) Swinging-down step in which, with a base end of the left beam of the jib 2 being rotatably connected on a horizontal shaft (one-side jib mounting shaft) 11 provided on the left side surface of a forward end portion of the boom, the jib 2 is swung down away from a base portion of the boom 1 about the horizontal shaft 11 from a folded position shown by phantom lines to a vertically suspended position shown by solid lines. The horizontal shaft 11 is provided at each of opposite side surfaces of the forward end portion of the boom 1, both horizontal shafts 11 extending horizontally from the opposite side surfaces of the boom 1 in opposite directions;
2) Twisting step in which the vertically suspended jib 2 is twisted in a direction of an arrow A with respect to an axial direction of the left beam of the jib 2; and
3) Swinging-up step in which the twisted jib 2 is swung up from the vertically suspended position to a position where the jib 2 is stretched out forwardly from the forward end portion of the boom 1.
The jib 2 can be returned from its stretched position to its folded position along the one side surface of the boom 1 in the reverse order of the above.
On the other hand, as shown in FIG. 6, in a jib down-folding type crane which holds a folded jib 2 having a left and a right beams on a lower surface of a telescopic boom 1 with the left and the right beams horizontally arranged (horizontal posture) 6, the jib 2 can be stretched out through the following two steps:
1) Swinging-down step in which, with base ends of the right and the left beams of the jib 2 rotatably connected on horizontal shafts (two-side jib mounting shaft) 11 on the opposite side surfaces of a forward end portion of the boom 1, the jib 2 is swung down away from a base portion of the boom 1 about the horizontal shafts 11 from a folded position shown by phantom lines to a vertically suspended position shown by solid lines. The horizontal shaft 11 is provided at each of opposite side surfaces of the forward end portion of the boom 1, both horizontal shafts 11 extending horizontally from the opposite side surfaces of the boom 1 in opposite directions; and
2) Swinging-up step which the vertically suspended jib 2 is swung up from the vertically suspended position to a position where the jib 2 is stretched out forwardly from the forward end portion of the boom 1.
The jib 2 can be returned from its stretched position to its folded position along the lower surface of the boom 1 in the reverse order of the above.
As steps commonly required in both the jib side-folding type crane and the jib down-folding type crane, there are the jib swinging-down step in which the jib 2 is moved from its folded position to its vertically suspended position in the stretching operation, and the jib hauling step in which the jib 2 is hauled from its vertically suspended position to its folded position.
As means for swinging down and hauling a jib, there has been conventionally used an auxiliary hoist wire rope (hereinafter referred to merely as a rope) 3 drawn out from an unillustrated winch provided on a crane main body in the following manner. The rope 3 is reeved from the forward end portion of the boom 1 through the forward end portion of the jib 2 to the base portion of the boom 1. The rope 3 may be reeved back from the base portion of the boom 1 to the forward end portion of the jib 2. The jib 2 can be swung down away from the base portion of the boom 1 or hauled thereto by unwinding or winding the rope 3.
However, the use of the conventional means for swinging down and hauling the jib invariably necessitates a series of cumbersome operations as follows:
a) to reeve the rope 3 a long distance along the boom 1 and the jib 2;
b) to engage and disengage an auxiliary hook with and from a free end of the rope 3 each time the crane operation is started or finished; and
c) to handle the rope 3 after the jib 2 is swung down or hauled.
Accordingly, it has been taking a great deal of work and time to stretch andd fold the jib 2, resulting in exceedingly low working efficiency.